The present application is based on Japanese Patent Application No. 2001-313983, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an optical sheet used as a transmission type screen member in a back projection type display device or as a viewing angle enlarging member in a liquid-crystal display device, and a display device using the optical sheet.
2. Description of the Related Art
Generally, a back projection type display device using a liquid-crystal display element or a dot matrix display element such as a digital mirror device has a projector for outputting image light formed through a display element, a reflecting mirror having a function of reflecting the image light outputted from the projector to change the direction of movement of the image light, and a transmission type screen for spreading the incident image light moderately to the observer side to display an image.
Generally, the transmission type screen includes a Fresnel lens sheet, and a lenticular lens sheet. The Fresnel lens sheet is an optical component having the same function as that of a convex lens. That is, the Fresnel lens sheet has a function of changing the direction of the image light outputted from the projector to the observer side to enlarge an optimal viewing range. On the other hand, the lenticular lens sheet has a function of distributing the limited image light outputted from the projector into an observer""s observation range effectively.
The lenticular lens sheet includes a plurality of cylindrical lenses arrayed in one direction, and a light absorbing layer disposed in other regions than light-condensing regions through the lenses for absorbing light inputted from the environment surrounding the lenticular lens sheet (hereinafter, the light being referred to as xe2x80x9cexternal lightxe2x80x9d). When focal points of the lenses are located on an observation surface of the screen, reflection of the external light is reduced without any loss of the image light ideally to thereby suppress lowering of contrast ratio under the bright environment.
Unexamined Japanese Patent Publication No. Hei. 10-260638 has disclosed a lenticular lens sheet having a transparent resin formed as lenses for achieving a wide viewing angle and a bright image, a light absorbing layer patterned, and a diffusing layer having a thickness of from 20 to 500 xcexcm and laminated between the transparent resin and the light absorbing layer.
Incidentally, a direct-vision liquid-crystal display device using TN (Twisted Nematic) liquid crystal or the like generally has viewing angle dependence in which luminance or chromaticity varies in accordance with the direction of observation. On the other hand, Unexamined Japanese Patent Publication No. Hei. 10-39769 has described an optical member which can eliminate the view angle dependence and reduce reflection of external light to thereby suppress lowering of contrast ratio under the bright environment when the optical member is disposed in front of a liquid-crystal display device.
The optical member is provided as a micro lens array sheet having a transparent substrate, a plurality of micro lenses constituted by fine convex surfaces and arrayed planarly on one surface of the transparent substrate, and a light absorbing layer disposed on the other surface of the transparent substrate and having opening portions located in places corresponding to apices of convex portions of the micro lenses.
Examples of the method for producing lenses of an optical member such as a lenticular lens sheet or a micro lens array sheet applied to a display device include: a method using a mold such as a metal mold, a resin mold or a stamper for transferring the shape of the mold; and a method of forming patterns from a resist material at intervals of a required pitch by photolithography and hot-melting the patterns.
A method using a metal mold for transferring the shape of the mold is most preferred as the method for producing a large-area lens array inexpensively. In this case, a transparent resin such as an ultraviolet-setting resin, a thermosetting resin or a thermoplastic resin is used as the lens material. The refractive index of the practical lens material is in a range of from about 1.48 to about 1.66.
FIG. 21 is a graph showing the relation between luminance and viewing angle characteristic in accordance with the refractive index n of a lens. FIG. 21 shows results of calculation in the case where rays of parallel light are incident on a hemispherical lens. In FIG. 21, the horizontal axis shows an emission angle (viewing angle), and the vertical axis shows a relative value of luminance.
As the refractive index n of a lens increases, the difference between luminance in a frontal direction (at an emission angle of 0 degrees) and luminance in an oblique direction decreases so that the viewing angle is widened.
If xe2x80x9cAt Panels session 1.5.2.Bxe2x80x9d in xe2x80x9cTCO (The Swedish Confederation of professional Employees) 99 requirementxe2x80x9d is now noticed as a standard for display devices, TCO 99 cannot be satisfied even in the case where the refractive index of a lens is 1.7, in accordance with the estimation. That is, an optical member capable of achieving a sufficient viewing angle cannot be obtained by use of only the refracting function of a lens using a practical lens material. It is therefore necessary to provide a diffusing layer for enlarging the viewing angle.
In the related art, there has been disclosed a lenticular lens sheet having a transparent substrate including lenses formed on one surface, a light absorbing layer, and a diffusing layer laminated between the transparent substrate and the light absorbing layer. In this case, the thickness of the diffusing layer is defined to be in a range of from 20 to 500 xcexcm from the point of view of preventing generation of a hot band and suppressing lowering of light transmittance but there is no particular consideration for display of a high-precision image.
The resolution of an image displayed on a general optical sheet such as a lenticular lens sheet or a micro lens array sheet depends on the pitch of lenses arrayed. As the pitch of lenses decreases, a higher-precision image can be displayed. It is therefore necessary that the pitch of lenses constituting an optical member is selected to be not larger than tens of microns in order to meet higher resolution of an image advanced with the popularization of HDTV (High Definition Television) in the future.
In this case, the following problem may arise if the diffusing layer is not sufficiently thin. FIG. 22 is a partly schematic sectional view of an optical sheet for explaining a problem in the case where the thickness d of the diffusing layer is not sufficiently small compared with the lens pitch.
Image light 2101 incident on an optical sheet is refracted by lenses 2000 so as to be condensed into a diffusing layer 2001. If the diffusing layer is not sufficiently thin, a part 2100 of light scattered by the diffusing layer is absorbed to a light absorbing layer 2002 to thereby cause a problem that transmittance is reduced.
On the other hand, it may be conceived that opening portions 2003 of the light absorbing layer are widened in order to suppress the reduction of transmittance. In this case, however, there arises another problem that lowering of contrast ratio under the bright environment occurs because absorption of external light to the light absorbing layer is reduced. It may be also conceived that the diffusing characteristic of the diffusing layer is reduced in order to suppress the reduction of transmittance. In this case, however, there arises a problem that the viewing angle is narrowed.
When, for example, the thickness of the diffusing layer is selected to be not smaller than 20 xcexcm in the condition that the pitch of lenses is selected to be 50 xcexcm in order to satisfy high-precision image display, there arises a problem that transmittance is reduced or contrast ratio is lowered.
Under such circumstances, an object of the invention is to provide an optical sheet which can exhibit high transmittance and have a viewing angle even in the case where the pitch of lenses is not larger than tens of microns in order to support high-precision image display and which can achieve high-contrast-ratio display even under the bright environment; and to provide a display device using the optical sheet.
In order to achieve the object, the gist of the invention is as follows. That is, an optical sheet has: a transparent base material; a fine lens array constituted by a plurality of fine unit lenses formed on a front surface of the transparent base material; a diffusing layer formed on a rear surface of the transparent base material opposite to the surface on which the fine lens array is formed; and a light absorbing layer formed on the diffusing layer and including fine opening portions substantially located at focal points of the unit lenses; wherein: the diffusing layer is made of a transparent medium containing transparent fine particles different in refractive index from the transparent medium; and the transparent fine particles have a mean particle size in a particle size range for maximizing a value of Q/R at each of wavelengths in a visible region when Q is a scattering sectional area per transparent fine particle, and R is a radius of each of the transparent fine particles.
Preferably, the transparent fine particles contained in the diffusing layer have a mean particle size in a particle size range for maximizing the value of Q/R at each of wavelengths in a visible region, or in a range which is near the particle size range and in which variation in the value of Q/R due to the wavelengths in the visible region is reduced.
As the value of Q/R increases, the light scattering characteristic of the diffusing layer increases exponentially. Hence, when the diffusing layer is formed in these conditions, required light scattering characteristic can be achieved though the diffusing layer is thinner.
Incidentally, the value of Q/R with respect to particle sizes of the transparent fine particles varies in accordance with wavelengths. Hence, when the mean particle size of the transparent fine particles is selected from a particle size range for maximizing the value of Q/R at wave lengths in a visible region, required light scattering characteristic can be achieved though the diffusing layer is thinner.
Particularly when transparent fine particles are used to have a mean particle size set as a particle size for maximizing the value of Q/R in a wavelength range of from 525 to 575 nm high in relative luminous efficiency, preferably at a wavelength of 550 nm, required scattering characteristic can be achieved by the diffusing layer smaller in thickness because light scattering characteristic actually sensed by the human being increases apparently.
Variation in the value of Q/R in accordance with wavelengths means variation in scattering characteristic in accordance with colors. That is, if a particle size in which the value of Q/R varies widely in accordance with wavelengths is used, there is the possibility that image quality may deteriorate because of variation in color in accordance with the angle of observation.
Therefore, in the optical sheet according to the invention, when the mean particle size of the transparent fine particles is selected to be in a particle size range for maximizing the value of Q/R or in a range which is near the particle size range and in which variation in the value of Q/R in accordance with wavelengths in a visible region is reduced, variation in color in accordance with the angle of observation can be suppressed.
In this manner, in the optical sheet according to the invention, there can be achieved a diffusing layer which is thin but has sufficient scattering characteristic.
When the diffusing layer is thin, there can be achieved an optical sheet which is high in transmittance, small in size of the opening portions of the light absorbing layer and small in reflection of external light even in the case where the pitch of lenses in the fine lens array is reduced in order to satisfy high-precision image display.
In the optical sheet according to the invention, the bottom surface of each of the unit lenses constituting the fine lens array is basically shaped like a hexagon so that the unit lenses are arranged so as to be closest packed. Hence, the area occupied by the unit lenses in the surface of formation of the fine lens array can be increased to thereby achieve higher transmittance.
Further, interior angles of the shape of the bottom surface of each of the unit lenses are all selected to be not lower than 90 degrees, preferably not lower than 100 degrees. Hence, the curvature of lens surfaces in corners of each unit lens and in regions near the corners is prevented from varying widely. Accordingly, the region effectively functioning as a lens can be widened to the utmost to thereby improve transmittance more greatly.
Further, the longitudinal length of the shape of the bottom surface in each of the unit lenses is selected to be larger than the transverse length thereof. Hence, directivity is given to light emitted from the optical sheet, so that the limited image light can be distributed to the observer effectively to thereby enhance luminance.
Accordingly, in an image display device using the optical sheet according to the invention, a high-contrast-ratio high-quality image can be obtained with high precision and high luminance at a wide viewing angle because of achievement of low-luminance black display even under the bright environment.
Particularly when the optical sheet according to the invention is disposed on the front surface of a direct-vision liquid-crystal display element while a backlight unit for outputting substantially parallel light rays is disposed on the rear surface side of the direct-vision liquid-crystal display element, only light in a limited range near the front of the liquid-crystal display element and permitting good image quality to be obtained can be spread isotropically by the optical sheet. Hence, there can be achieved a liquid-crystal display device for displaying a high-contrast-ratio image in a wide viewing angle range without variation in color tone and without inversion in gradation.
Features and advantages of the invention will be evident from the following detailed description of the preferred embodiments described in conjunction with the attached drawings.